The sphere slow down due to friction force between the surface of the sphere and the surface on that the sphere is rolling . The friction force acting against the motion of the sphere. Thats why it is slowed down. In fact not only a sphere, anything can not slow down untill a force act against it's motion.
All except C, feelings are not physical.
(10 waves) / (5 sec)
= (10/5) (wave/sec)
= 2 per sec
= 2 Hz .
